Low-alloy steels that are intended to be subjected to nitriding typically present a carbon content lying in the range 0.20% to 0.45% that serves to impart its mechanical properties to the core of the base material after heat treatment.
The surface properties of the steel, such as hardness, may be conferred by nitriding treatment, which consists in diffusing nitrogen into the ferritic phase and which causes submicroscopic nitrides to be precipitated from nitride-forming elements such as Cr, V, Mo, and Al that are present in solid solution in the treated steel.
Concretely, in a nitriding treatment, the steel may be treated at a temperature of about 500° C. with ammonia, which decomposes into cracked ammonia and reacts simultaneously with the iron of the steel. The ammonia causes a surface layer to be formed that is constituted by iron nitrides, referred to as a “compound” layer, from which nitrogen atoms diffuse towards the core of the part in order to form the diffusion layer.
For a low-alloy steel having nitride-forming elements, it is possible to observe two layers after nitriding: the compound layer at the surface constituted by iron nitrides, and the diffusion layer in which submicroscopic nitride precipitates are dispersed that lead to the increase in hardness that is observed in the nitrided layer.
The depth of the nitrided layer may vary, depending on nitriding conditions and on the intended applications, in the range 0.05 millimeters (mm) to 1 mm.
Nevertheless, nitriding treatment remains treatment that is relatively lengthy to perform.
It would be desirable to further improve the surface properties of nitriding layers and to reduce the time required for nitriding treatment.
There thus exists a need to improve the surface properties, in particular the hardness, of nitrided layers formed at the surface of low-alloy steel parts.
There also exists a need to accelerate the apparent speed of treatment for nitriding low-alloy steel parts, i.e. for reducing the time needed to obtain a significant increase in hardness to a given depth, e.g. an increases of at least 200 on the Vicker's hardness scale (HV) compared with the hardness so the steel in the core.